The gonadotrophins are a group of heterodimeric glycoproteins including follicle stimulating hormone (FSH), luteinising hormone (LH) and chorionic gonadotrophin (CG). These hormones regulate gonadal function in the male and female. Each of these hormones is composed of two non-covalently linked subunits: an alpha-subunit, which is common to FSH, LH and hCG, and a beta-subunit, which is unique to each of them, and which confers biological specificity to each hormone.
In all of the gonadotrophins, each sub-unit has asparagine-linked (N-linked) oligosaccharides side chains. In the common alpha-subunit of the human hormones, these are attached at positions 52 and 78. In both human FSH and CG, two N-linked oligosaccharide side chains are attached to the beta-subunit, at positions 7 and 24 in FSH, and positions 13 and 30 in hCG. In human LH, one oligosaccharide is attached at position 30 of the beta-subunit. hCG has additionally four serine-linked (O-linked) oligosaccharide side chains, present in the carboxyl terminal portion (CTP).
The gonadotrophins play crucial roles in the reproductive cycle, and their use is essential for assisted reproductive techniques (ART), such as in vitro fertilisation (IVF), IVF in conjunction with intracytoplasmic sperm injection (IVF/ICSI) and embryo transfer (ET), as well as for ovulation induction (OI) in anovulatory patients undergoing in vivo fertilisation either naturally or through intrauterine insemination (IUI).
ART is typically carried out using controlled ovarian hyperstimulation (COH) to increase the number of female gametes. Standard regimens for COH include a down-regulation phase in which endogenous gonadotrophins are suppressed by administration of a gonadotrophin releasing hormone (GnRH) agonist followed by a stimulatory phase in which follicular development (folliculogenesis) is induced by daily administration of FSH, usually at about 150-225 IU/day. Alternatively stimulation is started after spontaneous or induced menstruation while preventing the occurrence of an ill-timed LH surge by administration of a GnRH-antagonist (typically starting around day six of the stimulatory phase). When there are at least 3 follicles>16 mm (one of 18 mm), a single bolus of hCG (5-10,000 IU) is given to mimic the natural LH surge and induce ovulation. Oocyte recovery is timed for 36-38 hours after the hCG injection.
OI is typically carried out with daily administration of FSH at a dose of about 75-150 IU/day. Down-regulation with GnRH agonists or antagonists may be used, although less frequently than in the ART indication. hCG is given to mimic the LH surge prior to in vivo fertilisation which is achieved either through regular intercourse or IUI.
The typical regimens described above for ART and OI require daily injections of gonadotrophins over a prolonged period, i.e. for an average of 10 days, and up to 21 days in some patients. The development of FSH preparations of increased efficacy would permit the daily dosage of FSH to be decreased, and/or permit a shortening of the treatment period (i.e. fewer injections), and/or allow injections to be given less frequently. This would render ART and OI regimens more convenient and patient-friendly.
Furthermore, ART using in vitro fertilisation is fraught with possible mishaps. For example, not every follicle will produce a viable oocyte, not every viable oocyte will be successfully fertilised, and some embryos may not be viable. Moreover, once viable embryos are selected, transfer to the uterus and implantation may not be successful. In order to maximise the chances of a live birth it is therefore desirable to stimulate the growth and maturation of several follicles, to ensure the collection of multiple oocytes.
In the indication of OI, in contrast, the objective is to obtain not more than three and preferably one dominant follicle (to avoid multiple pregnancies).
Some patients undergoing ART and OI present a reduced number of growing follicles when treated with conventional FSH preparations. This is a limiting factor for success when undergoing ART, in that it limits the number of embryos available for transfer and/or cryopreservation. It can also be a limiting factor for success in patients undergoing IUI, where obtaining more than one follicle is important. Patients presenting this type of response include patients above about 33-35 years old, patients with elevated base-line FSH, elevated base-line oestradiol or reduced base-line inhibin b.
In the male, spermatogenesis is dependent on stimulation of Sertoli cells by FSH. FSH deficiency results in oligospermia, and hence infertility. The treatment of male infertility with conventional FSH preparations requires FSH injections three times a week for up to 18 months.
Modified FSH molecules, bearing additional glycosylation sites are described, in WO 01/58493 (Maxygen). One example of a modified FSH molecules is tested and reported to have a reduced bioactivity and an increased in vivo half-life.
The development of FSH preparations with enhanced ability to stimulate folliculogenesis, is an ongoing need. There is also an existing need for new FSH preparations to treat patients with a diminished response to FSH. Also desirable are FSH preparations of longer half-life, permitting shorter treatment protocols and/or decreased cumulative doses and/or less frequent dosing, for ART, OI and male infertility.